Coating process for a superalloy article

ABSTRACT

THE COATING OF NICKEL- AND COBALT-BASE ALLOY ARTICLES TO IMPROVE BOTH THERMAL FATIQUE AND SULFIDATION RESISTANCE OF SAID ARTICLES. THE COATING IS APPLIED BY PACK CEMENTATION IN AN INERT OR NON-REACTIVE ATMOSPHERE OR IN A VACUUM, USING AS A PACK POWDER AN ALLOY OF ALUMINUM AND ONE OR MORE RARE EARTH ELEMENTS.

United States Patent Office 3,794,5 l l Patented Feb. 26, 1 974 3,794,511 COATING PROCESS FOR A SUPERALLOY ARTICLE Sanford Baranow, Woodbridge, Conn., assignor to Avco Corporation, Stratford, Conn.

No Drawing. Continuation of abandoned application Ser. No. 686,852, Nov. 30, 1967. This application Oct. 22, 1971, Ser. No. 191,819

Int. Cl. C23c 9/02 US. Cl. 1l722 Claims ABSTRACT OF THE DISCLOSURE The coating of nickeland cobalt-base alloy articles to cooled to room temperature. Metallographic and microprobe examination of the resulting coating derived discloses an outer zone rich in Misch Metal overlaying a zone chiefly of Ni Al Below the Ni Al zone is a layer of improve both thermal fatigue and sulfidation resistance of NiAl under which is the article being coated. Comparative articles The coating is pp y P k cementation testing of these specimens and specimens which had been inert non-reactive atmosphere 111 a Vacuum, aluminized with no rare earth present showed a definite 118mg as a P Powder an alloy of alumlnum one superiority of the rare earth aluminized articles, espemore rare earth elementscially in resistance to sulfidation, as compared with the aluminized articles. Thi i a gontinuation f application 5 N 686,852 Similar improvements were obtained with articles made filed Nov. 30, 1967, and now aband d, from high nickel and high cobalt alloys of the following This invention relates to improvements in the properties nominal compositions:

Sr 00 Zr Fe M0 w Al Sb Ti Ta s B N1 .10 1 4 2 5.5 1 2 .18 .2 Bal. 24.5 1 7.4 .5 10.5 5-30 15-65 0-10 345 Up m5 Up to3 Up to 0.5 10-80 of articles made of nickel-base or cobalt-base superalloys. or any of the alloys in US. Pat. 3,096,160, or any other More particularly, it relates to the coating of such articles of the niokelor cobalt-base superalloys presently known by cementation in a powder pack consisting essentially of in the art. an alloy of aluminum and at least one rare earth element, The coating powder used in the example is preferred, the cementation being carried out in a non-reactive atbecause Misch Metal is one of the least expensive sources mosphere. The rare earth element(s) alloyed with the of rare earths. Alloys or mixtures comprising between 55 aluminum are those exhibiting a relativel high vapor and 73% Misch Metal, or cerium or lanthanum or other pressure and the powder composition is such that no meltrare earth metals and between and 27% A1 are preing occurs during the coating process. ferred to alloys containing higher percentages of A], such The procedure: whereby articles are provided with an as those described in US. Pat. 2,980,529, i.e., those conouter coating for the purpose of enhancing oxidation re- 0 taining 7096% Al. sistance or other properties involves diffusion of one or Instead of a vacuum furnace, the heating can be carried more elements into the article and are well known in the Out in any controlled atmosphere furnace in which an art. inert atmosphere (argon, helium or other noble gas) or Recent patents describing the application of such coata non-reactive atmosphere (hydrogen) is provided and inings to superalloys include the following: Wachtell et al., 45 which the temp can be maintained at a y desired 3,073,015; Puyear et al., 3,079,276; and Puyear, 3,086,- value between 1500 F. and 2000 F. 160. When rare earth elements such as yttrium are alloyed The disclosures of these patents are incorporated herein with A the low Vapor Pressure of the r ar h l t by reference, as representative of the prior art and for may be overcome by including a small amount of NH Cl their definitions of superalloys and other terms in the or other volatile halide in the pack in order to effect transart. port and diffusion of the yttrium into the substrate.

The present invention provides articles with enhanced What is claimed is: thermal fatigue and sulfidation resistance, as compared 1. A process for coating nickel-base superalloy articles with articles not treated in accordance with the invention, to provide them with improved sulfidation resistance and or with aluminized articles, i.e. articles provided with i p ve thermal fatigue Which Comprises: coatings of aluminum diffused into or alloyed with the embedding said articles in a pack which consists essensurface layers of the article. tially of particles of a powdered aluminum-rare earth Briefly, the process of the invention involves the folalloy containing between 55 and 73% by weight of lowing: rare earth and between 45 and 27% by weight of Embedding articles to be coated in a mass of powdered aluminum; aluminumrare earth alloy; heating said articles While so embedded to a tempera- Heating the article while so embedded to a temperature ture between 1500 F. and 1900 F. while maintainbetween 1500 and 1900 F.; ing said articles in a non-reactive atmosphere;

Maintaining the pack in a non-reactive atmosphere maintaining said temperature for between 2 and 8 hours while it is heated for a desired time interval, usually beand thereafter permitting said articles to cool; tween 2 and 8 hours; and recovering the resultant coated superalloy articles Cooling the article; and each bearing a surface into which said rare earth- Recovering the coated article from the pack. aluminum alloy has diffused; The invention will be better understood from the ex the resulting superalloy articles comprising a superample which follows: alloy base exhibiting a varying composition from the Articles made of Inco 713 Calloy, an alloy whose nominal composition is 13% Cr, 5.75% Al, 0.75% Ti, 4.5%

outer surface thereof to the inner core thereof as a result of said process, namely: a first zone adjacent 3 to the unaffected core of said articles consisting principally of NiAl and being integral with said superalloy article, a second zone consisting essentially of Ni2Al3 and disposed between said first zone and a third zone constituting the outer surface of the coated article, said third zone consisting essentially of an alloy of aluminum, rare earth and superalloy and being integral with said article. 2. The process of claim 1 wherein the powdered pack contains a substantial amount of a volatile halide.

3. The process of claim 2 wherein the volatile halide is ammonium chloride.

4. The process of claim 1 wherein the rare earth in said powdered alloy is supplied by Misch metal.

5. The process of claim 1 wherein the heating step is conducted while the article and the powder pack are in a vacuum furnace.

References Cited UNITED STATES PATENTS 6/1969 Ortner et al 75-l38 X Puyear et a1 117-131 X Samuel 117-107.2 PX Wachtell et al. 117107.2 PX Becket 117131 X Howe 117119 X Samuel 117--22 Mayer et al. 117107.2 P Runnalls 75152 X Beghi et al. 75138 Gilson 117-131 X Schonfeld et al. 75-138 X Knapp et al. 75--58 X FOREIGN PATENTS Great Britain 11722 US. Cl. X.R. 

